This invention relates to a tooling fixture to support a workpiece. In particular, this invention relates to a hydraulic fixture for supporting a flexible substrate such as a printed circuit board to be used in a screen printing machine or placement machine.
Fabrication of large volumes of electronic circuitry is economically accomplished using automated equipment. According to one known process a viscous solder paste is applied to selected areas of a printed circuit board through a stencil in a screen printing machine. Electronic components are then placed on the applied solder paste using a placement machine (also known as a pick-and-place machine). The printed circuit board and components are then heated in a reflow oven to a temperature sufficient to melt the paste, causing it to flow over the leads of the components and adjacent areas of the printed circuit board, thereby forming solder joints and completing the electronic circuit.
Within the screen printing machine a pair of conveyor belts support the printed circuit board by its edges. The conveyor belts move the board into the machine and position it relative to the stencil. A tooling fixture below the underside of the board is raised so that it presses upward on the lower surface of the board t) provide support for the board during the printing process. The board and tooling are then raised until the top surface of the board contacts the stencil. A printhead applies the solder paste to the top surface of the stencil and presses the paste against the stencil. Pressure applied by the printhead forces solder through holes in the stencil and onto the top surface of the board. The tooling fixture prevents the board from flexing away from the stencil as pressure is applied by the printhead.
Conveyor belts also move the circuit board through the placement machine. After solder is applied to the board by the screen printing machine, the board is moved into the placement machine and positioned relative to a robotic pick-and-place head. A tooling fixture located below the board is raised to support the underside of the board. Under the control of a program that defines the proper location of each of the components of the circuit, the pick-and-place head sequentially presses each component onto its proper location on the printed circuit board. The tooling fixture prevents the board from flexing as pressure is applied by the pick-and-place head.
Where components are placed on only one surface of the board, the tooling fixtures merely have to support a flat surface, and only slight modifications to the fixtures need to be made to accommodate boards with different linear dimensions. However, in order to better utilize the surface area of printed circuit boards and produce more compact circuitry, components are often placed on both surfaces of the board. In such cases, after components have been applied to one side of the board, the board is then turned over and components are placed on the other side of the board. Consequently, a second tooling fixture is required that conforms to the three-dimensional surface created by the components placed on the board. Since each different circuit layout has a different three-dimensional shape, this second tooling fixture must be customized for each layout.
One way to provide a customized, three-dimensional tooling fixture is to manually assemble blocks on the top surface of the tooling fixture that are arranged to contact the portions of the board not populated by components. However, this method is time consuming because an operator must reposition the blocks each time a different board is processed. This method is particularly cumbersome when a large number of different types of circuit boards are fabricated on the same assembly line.
Another method is to provide an array of electronically addressable posts that are either retracted in positions where a component is attached or extended where they can contact the board surface to provide support. A computer program directs the tooling fixture to configure the posts to conform to each printed circuit board that is fabricated. This method provides a faster means to change from one tooling configuration to another than the manual assembly of blocks. However, a separate computer database must be created for each layout of each board. Further, since no support is provided to the board in areas covered by components, certain portions of the board may be inadequately supported. Unsupported portions of the board may flex during processing resulting in misalignment between the stencil or the pick-and-place head and the board.
Yet another method is to provide a robot mechanism to automatically take support pillars from a reservoir and place them in the required positions to support the board. This method is faster and more consistent than manually placing support blocks, but needs a separate computer database for each board type to define where pillars should be placed and can only provide support where there is clear space on the board for the pillar.
Prior to using a customized fixture with a new board layout, an operator must carefully inspect the fit between the tooling fixture and the board to make certain that misalignment and dimensional tolerances do not cause an extended post, positioned pillar, or a manually positioned block to contact a component. Such contact will cause the tooling fixture to flex the board upward resulting in misplacement of the solder paste or components. More importantly, flexure of the board upward against the stencil can cause damage to the stencil or printhead of the screen printing machine.
Inspection of the tooling fixture each time a new type of board is fabricated complicates the manufacturing process and lowers throughput. Further, if the operator fails to properly inspect the fit between the board and the fixture, large numbers of misaligned boards may be produced or costly damage to the equipment may result.
Furthermore, even when a fabrication line produces only a single type of printed circuit board, each side of the board requires a different tooling fixture surface. A xe2x80x9cflatxe2x80x9d tooling fixture supports the unpopulated side of the board while the first side is screen printed and populated with components, and then a customized fixture supports the now populated side while the second side of the board is screen printed and populated. Even this tooling change reduces the throughput of the fabrication line.
In view of the above-identified problems with know methods for forming tooling fixtures, it is an object of the present invention to provide a tooling fixture that conforms to an uneven surface without having been configured beforehand.
It is another object of the invention to provide a tooling fixture that supports an uneven surface formed by a printed circuit board with components fixed thereon within a screen printing machine.
It is yet another object of the invention to provide a tooling fixture that supports an uneven surface formed by a printed circuit board with components fixed thereon within a placement machine.
It is still a further object of the invention to provide a tooling fixture that adapts to the surface of populated and unpopulated printed circuit boards.
It is still a further object of the invention to provide a tooling fixture that supports a workpiece using an array of hydraulic cylinders driving a respective array of pistons.
It is still a further object of the invention to provide a tooling fixture that supports a workpiece using an array of bearing and seal assemblies for directing an array of support rods to be driven.
According to one aspect of the present invention a block is provided with a plurality of cylinders. Each cylinder surrounds a piston that drives a rod extending out of the top surface of the block. The cylinders are connected with a pressure sensor and an accumulator. A controllable valve is connected between the accumulator and the cylinders. Opening the valve allows hydraulic fluid to flow between the cylinders and the accumulator. The pressure sensor monitors the hydrostatic pressure of the hydraulic fluid in the cylinders. A compressed air source is connected to the accumulator through a further valve, such that when this valve is opened a force is generated to drive fluid into the cylinders, thereby causing the pistons to extend from the block. Similarly a vacuum source is also connected to the accumulator through a third valve, such that with this valve open and the compressed air valve closed, a vacuum is applied to the accumulator to draw fluid from the cylinders, thereby causing the pistons to retract into the block. The pressure sensor and the valve are connected with a controller that opens and closes the valve in response to a control sequence and the hydrostatic pressure in the cylinders.
The tooling fixture according to this aspect conforms to an uneven surface as follows. The controller opens the fluid valve and the vacuum valve to draw fluid from the cylinders causing the pistons to retract the rods into the block. A workpiece is positioned above the fixture. The workpiece can be substantially flat or can have three-dimensional structures fixed thereon. According to one embodiment, the workpiece is a printed circuit that is either populated with components or else unpopulated. The controller closes the vacuum valve and opens the compressed air valve, forcing fluid into the cylinders and raising the rods. When all of the rods have contacted the surface of the workpiece, the pressure of the fluid sensed by the pressure sensor rises. When this pressure reaches a predetermined threshold, the controller closes the fluid valve and the compressed air valve, and the positions of the rods are fixed. Force applied to the top surface of the workpiece, for example by the printhead of a screen printing machine, is opposed by the rods and deflection of the workpiece is prevented.
According to another embodiment of the present invention a tooling fixture module is provided with a plurality of cylinders. Each cylinder surrounds a piston wherein a portion of the piston extends out of the top surface of the module. The cylinders are connected with a fluid reservoir and pressure source. A control valve assembly is connected between the fluid reservoir and the cylinders. Opening the valve permits fluid to flow between the cylinders and the fluid reservoir, providing for raising or lowering the pistons.
The tooling fixture according to this aspect conforms to an uneven surface as follows. Pneumatic pressure acts on a main piston, located within a fluid source. The fluid then acts on a control valve while in its open position in order to provide fluid to the cylinders of the module, causing a portion of the pistons to extend out of the module body. A workpiece is positioned above the tooling fixture. The workpiece can be substantially flat or can have three-dimensional structures fixed thereon. This workpiece can be a printed circuit that is either populated with components or else unpopulated. Each piston that makes contact with the workpiece will remain in this contact position while the other pistons continue to extend out of the module. After all of the pistons have made contact with the workpiece, the control valve closes, preventing the fluid from further entering or leaving the cylinders, thus effectively locking the pistons in a fixed position. Force applied to the top surface of the workpiece, for example, by the printhead of a screen printing machine is opposed by each piston, and deflection of the workpiece is prevented. In order to lower the pistons, the control valve opens, allowing the fluid to exit the cylinders through the valve, and the pistons will retract back into the module.
According to another embodiment of the invention, the fluid source can be in the form of a hydraulic tube containing a first main piston therein for forcing fluid towards the control valve and into the cylinders causing the pistons to move upward extending a portion thereof out of the surface of the module.
According to another embodiment of the invention, the hydraulic tube can be replaced with a fluid reservoir.
According to a further embodiment of the invention, a second fluid reservoir is used in conjunction with the first fluid reservoir to force fluid into cylinder sections above the pistons. A second pneumatic pressure source applies pressure directly to the control valve and to the second fluid reservoir simultaneously. Consequently the control valve is opened and allows fluid to flow from the cylinder sections below the pistons into the first fluid reservoir while fluid from the second reservoir is forced into the cylinder sections above the pistons, causing a portion of the pistons to retract into the module. Similarly, a first pneumatic pressure can be applied to the first fluid reservoir in order to force fluid through the control valve and into cylinder sections below the pistons. This will force the fluid contained in the cylinder sections above the pistons back into the second fluid reservoir, thus raising a portion of the pistons out of the module.
According to a further embodiment of the invention the cylinder and piston assemblies are replaced with rod assemblies each containing support rods, integrally connected at their bottom portions by a large singular cavity. Applying a first pneumatic pressure to a main piston within a fluid reservoir will force fluid through a one-way path in a control valve and into the large singular cavity. The fluid in the large singular cavity will force the support rods to extend out of the module until each of the rod makes contact with the workpiece. To retract the support rods, a second pneumatic pressure is applied to the control valve assembly to open a flow path allowing the fluid to flow from the large singular cavity to the fluid reservoir.
According to another embodiment of the invention the control valve is replaced with a permanent aperture between a single reservoir and a large singular cavity below a series of rod assemblies. The single reservoir includes a main piston with main rod connected thereto it. A first pneumatic pressure source acts on the main rod and main piston to extend the combination towards the fluid within the fluid reservoir, forcing the fluid within the fluid reservoir through the permanent aperture and into the large singular cavity, thus raising the rods. A second pneumatic pressure source acts on the main rod and main piston to retract the combination away from the fluid within the fluid reservoir, drawing the fluid out of the large singular cavity and back into the fluid reservoir, thus retracting the rods. A third pneumatic pressure source acts on a clamp provided to prevent or allow the main piston and main rod combination within the reservoir to move. In effect, when pressure is applied to the clamp, the main rod is disengaged and permitted to move upon application of pressure applied to it from either the first or second pneumatic pressure source.
It is to be understood that both the foregoing general description and the following detailed description is exemplary and explanatory and is intended to provide further explanation of the invention claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.
Further objects, features, and advantages of the invention will become apparent from the detailed description and drawings that follow.